The use of heavy mineral correlation for determining the source of impact ejecta

A Manicouagan distal ejecta case study

Scott Neil Thackrey, Gordon Mark Walkden, A. Indares, A. Horstwood, S. Kelley, R. Parrish

Research output: Contribution to journalArticle

14 Citations (Scopus)

Abstract

Abstract
Due to the nature of distal impact ejecta layers on Earth, preserved deposits are rare and are commonly altered by diagenetic/weathering processes. To establish the source of ejecta, trace element signatures of impact glasses have typically been used to establish the source crater of a deposit. However, in many cases, impact glasses in ejecta deposits are devitrified and altered to clays resulting in loss of original melt and original trace element composition. This is the case for the Late Triassic ejecta deposit of SW Britain where impact melt spherules have been completely altered to clay. This Late Triassic ejecta deposit was originally believed to be derived from two possible sources, the Rochechouart or Manicouagan impact structures. To accurately establish the source of this ejecta deposit an alternative correlation technique was developed using garnet major oxide data and radiometric whole grain fusion Ar–Ar dating of shocked biotite (U–Pb dating of zircon was also used but proved inconclusive). Radiogenic dating of shocked biotites (observed exclusively in the ejecta deposit) yielded ages consistent with the Grenvillian target rocks at Manicouagan and excluded Rochechouart as a potential source. Garnet major oxide compositions of garnets in the ejecta deposit were directly compared to garnets from the Manicouagan target rocks and impactite rocks. A strong garnet composition signature correlation between the samples from Manicouagan and the ejecta deposit provides convincing evidence that Manicouagan is the source of the SW Britain ejecta deposit. Furthermore, we suggest that heavy mineral correlation techniques should be considered in future studies as a correlation tool for establishing the source of ejecta.

Keywords: correlation; heavy minerals; ejecta; Manicouagan; alteration
Original languageEnglish
Pages (from-to)163-172
Number of pages10
JournalEarth and Planetary Science Letters
Volume285
Issue number1-2
DOIs
Publication statusPublished - 30 Jul 2009

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heavy mineral
ejecta
Minerals
Deposits
minerals
deposits
Garnets
garnets
garnet
dating
Rocks
Trace Elements
Oxides
rocks
trace elements
clays
Chemical analysis
Triassic
impactite
Glass

Keywords

  • correlation
  • heavy minerals
  • ejecta
  • Manicouagan
  • alteration

Cite this

The use of heavy mineral correlation for determining the source of impact ejecta : A Manicouagan distal ejecta case study . / Thackrey, Scott Neil; Walkden, Gordon Mark; Indares, A. ; Horstwood, A. ; Kelley, S.; Parrish, R. .

In: Earth and Planetary Science Letters, Vol. 285, No. 1-2, 30.07.2009, p. 163-172.

Research output: Contribution to journalArticle

Thackrey, Scott Neil ; Walkden, Gordon Mark ; Indares, A. ; Horstwood, A. ; Kelley, S. ; Parrish, R. . / The use of heavy mineral correlation for determining the source of impact ejecta : A Manicouagan distal ejecta case study . In: Earth and Planetary Science Letters. 2009 ; Vol. 285, No. 1-2. pp. 163-172.
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N2 - Abstract Due to the nature of distal impact ejecta layers on Earth, preserved deposits are rare and are commonly altered by diagenetic/weathering processes. To establish the source of ejecta, trace element signatures of impact glasses have typically been used to establish the source crater of a deposit. However, in many cases, impact glasses in ejecta deposits are devitrified and altered to clays resulting in loss of original melt and original trace element composition. This is the case for the Late Triassic ejecta deposit of SW Britain where impact melt spherules have been completely altered to clay. This Late Triassic ejecta deposit was originally believed to be derived from two possible sources, the Rochechouart or Manicouagan impact structures. To accurately establish the source of this ejecta deposit an alternative correlation technique was developed using garnet major oxide data and radiometric whole grain fusion Ar–Ar dating of shocked biotite (U–Pb dating of zircon was also used but proved inconclusive). Radiogenic dating of shocked biotites (observed exclusively in the ejecta deposit) yielded ages consistent with the Grenvillian target rocks at Manicouagan and excluded Rochechouart as a potential source. Garnet major oxide compositions of garnets in the ejecta deposit were directly compared to garnets from the Manicouagan target rocks and impactite rocks. A strong garnet composition signature correlation between the samples from Manicouagan and the ejecta deposit provides convincing evidence that Manicouagan is the source of the SW Britain ejecta deposit. Furthermore, we suggest that heavy mineral correlation techniques should be considered in future studies as a correlation tool for establishing the source of ejecta. Keywords: correlation; heavy minerals; ejecta; Manicouagan; alteration

AB - Abstract Due to the nature of distal impact ejecta layers on Earth, preserved deposits are rare and are commonly altered by diagenetic/weathering processes. To establish the source of ejecta, trace element signatures of impact glasses have typically been used to establish the source crater of a deposit. However, in many cases, impact glasses in ejecta deposits are devitrified and altered to clays resulting in loss of original melt and original trace element composition. This is the case for the Late Triassic ejecta deposit of SW Britain where impact melt spherules have been completely altered to clay. This Late Triassic ejecta deposit was originally believed to be derived from two possible sources, the Rochechouart or Manicouagan impact structures. To accurately establish the source of this ejecta deposit an alternative correlation technique was developed using garnet major oxide data and radiometric whole grain fusion Ar–Ar dating of shocked biotite (U–Pb dating of zircon was also used but proved inconclusive). Radiogenic dating of shocked biotites (observed exclusively in the ejecta deposit) yielded ages consistent with the Grenvillian target rocks at Manicouagan and excluded Rochechouart as a potential source. Garnet major oxide compositions of garnets in the ejecta deposit were directly compared to garnets from the Manicouagan target rocks and impactite rocks. A strong garnet composition signature correlation between the samples from Manicouagan and the ejecta deposit provides convincing evidence that Manicouagan is the source of the SW Britain ejecta deposit. Furthermore, we suggest that heavy mineral correlation techniques should be considered in future studies as a correlation tool for establishing the source of ejecta. Keywords: correlation; heavy minerals; ejecta; Manicouagan; alteration

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